Neocortex is the hallmark of the mammalian brain. One of the most obvious features of its structure is also basic to the plan of the vertebrate body, i.e., bilateral symmetry. Integration of bilateral sensory information in the two hemispheres is achieved in two ways: (1) relay to the cortex from thalamic nuclei receiving bilateral sensory inputs and (2) interconnection of the hemispheres by commissures. Classical theories of the evolution of neocortex have been based in part on comparisons of the appearance of these commissures in mammals and reptiles. However, recent experimental evidence suggests that some of the classical views may be revised. In particular, the corpus striatum, or dorsal ventricular ridge, of the reptile telencephalon, thought to be homologous to basal ganglia, now appears to correspond to parts of neocortex, on the basis of its afferent connections. A portion of general cortex in reptiles may also be homologous to other regions of neocortex. However, we know very little about the efferent connections of the general cortex or the dorsal ventricular ridge (DVR). As reptiles do not appear to have a corpus callosum, identification of commissural connections of the DVR would have important bearing on the origin of this massive neocortical commissure. It is therefore proposed to investigate both aspects of the problem of bilateral symmetry in regard to neocortical evolution. I specifically propose (1) to continue current studies of bilateral versus ipsilateral sensory projections to the thalamus of reptiles and (2) to investigate the efferent projections of the DVR and cortex, both commissural and descending. Standard experimental anatomical techniques for silver impregnation of anterograde axonal degeneration will be used. It is hoped that comparisons of these projections with the known sensory pathways and neocortical projections in mammals will then clarify a number of problems regarding the evolutionary origin of both neocortex and the corpus callosum, making functional comparisons of morphologically divergent structures possible.